Filter Compatibility: Compatible with Honeywell HRF-C1 and HRF-C2 air filters. Maintenance: Recommended to change this filter once every six (6) months under normal conditions, and more often under harsh air conditions. Multipack: Each package contains two (2) replacement filters. Filter-Monster branded compatible with Honeywell C Filter (HRF-C1) removes smoke, dust, pollen, pet dander, mold spores, and dust mite debris from the air.